[electro-magnetic interference protection circuit and method for a clock buffer]

ABSTRACT

The present invention provides an electro-magnetic interference protection circuit for a clock buffer. The present invention not only uses a ferrite bead that is serially connected with the power source of the clock buffer to protect the clock buffer from electro-magnetic interference. The ferrite bead also connects in parallel with a capacitor to reduce the distortion of a clock signal out-putting by the clock buffer due to the ferrite bead, thereby making the communication devices work properly.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of Taiwanapplication serial no. 92115368, filed on Jun. 6, 2003.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] This invention generally relates to an electro-magneticinterference protection circuit and a method, and more particularly toanelectro-magnetic interference protection circuit and a method for aclock buffer.

[0004] 2. Description of Related Art

[0005] As the technology advances, Internet becomes one of the mostimportant resources for people to obtain knowledge and is a powerfultool for commercial transactions. Modem has been used in personalcomputers for Internet connection. Today, high speed Internet solutionssuch as ADSL products are widely used for better quality. In thosecommunication devices, the transceiver ICs are driven by clock signalsthat are distributed from clock buffers. Hence, how to protect the clockbuffers from electro-magnetic interference (“EMI”) is an importantissue.

[0006] Conventionally, a ferrite bead is serially connected to the powersource of the clock buffer for EMI protection. How ever, because of itsinductor characteristic, the ferrite bead also affects the waveform ofthe clock signal. FIG. 1 is a timing diagram of the input clock signalclk-in and the output clock signal clk-out when the ferrite bead isserially connected to the power source of the clock buffer. As shown inFIG.1, the waveform of the output clock signal clk-out is affected bythe ferrite bead. Hence, the communication devices cannot work properlybecause the highest voltage level and the lowest voltage level of theoutput clock signal clk-out are too close to be deter-mined. Althoughthe ferrite bead may protect the clock buffer from EMI, it affects theoutput clock signal of the clock buffer. Therefore, a solution for EMIprotection without affecting the output clock signal of the clock bufferis needed.

SUMMARY OF INVENTION

[0007] An object of the present invention is to provide anelectro-magnetic interference protection circuit and method for a clockbuffer to protect the clock buffer from EMI and to keep thecommunication device work properly.

[0008] In accordance with the above objects and other advantages of thepresent invention, an electro-magnetic interference protection circuitfor a clock buffer is provided. The electro-magnetic interferenceprotection circuit comprises a clock buffer for receiving a clock signaland buffer-outputting the clock signal; a ferrite bead, the ferrite beadserially being connected to the power source of the clock buffer toprotect the clock buffer from electro-magnetic interference (e.g., thepower source); and a capacitor connected in parallel with the ferritebead to reduce the distortion of the clock signal due to the ferritebead.

[0009] In an embodiment of the present invention, the ferrite bead”simpedance is at a maximum at the frequency of the electro-magneticinterference. For example, when the frequency of the electro-magneticinterference is 125 MHz, the ferrite bead” s impedance is at a maximumat 125 MHz to protect the clock buffer from EMI.

[0010] In an embodiment of the present invention, the impedance of thecapacitor is at a minimum at the frequency of the clock signal. Forexample, when the frequency of the clock signal is 25 MHz, the impedanceof the capacitor is at a minimum at 25 MHz to reduce the distortion of aclock signal outputting by the clock buffer due to the ferrite bead.

[0011] The present invention provides an electro-magnetic interferenceprotection method for a clock buffer. The method comprises the steps of:connecting a ferrite bead in serial with the power source of the clockbuffer to protect the clock buffer from electro-magnetic interference(e.g., the power source); and connecting a capacitor in parallel withthe ferrite bead to reduce the distortion of a clock signal outputtingby the clock buffer due to the ferrite bead.

[0012] In an embodiment of the present invention, the ferrite bead”simpedance is at a maximum at the frequency of the electro-magneticinterference. For example, when the frequency of the electro-magneticinterference is 125 MHz, the ferrite bead” s impedance is at a maximumat 125 MHz to protect the clock buffer from EMI.

[0013] In an embodiment of the present invention, the impedance of thecapacitor is at a minimum at the frequency of the clock signal. Forexample, when the frequency of the clock signal is 25 MHz, the impedanceof the capacitor is at a minimum at 25 MHz to reduce the distortion of aclock signal outputting by the clock buffer due to the ferrite bead.

[0014] Accordingly, the present invention not only uses a ferrite beadto protect the clock buffer from electro-magnetic interference, but alsoconnects a capacitor in parallel with the ferrite bead to reduce thedistortion of a clock signal output by the clock buffer due to theferrite bead, thereby making the communication devices work properly.

[0015] The above is a brief description of some deficiencies in theprior art and advantages of the present invention. Other features,advantages and embodiments of the invention will be apparent to thoseskilled in the art from the following description, accompanying drawingsand appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG.1 is a timing diagram of the input clock signal clk-in andthe output clock signal clk-out when the ferrite bead is connected inserial with the power source of the clock buffer.

[0017]FIG.2. is an embodiment of an electro-magnetic interferenceprotection method for a clock buffer in accordance with the presentinvention.

[0018] FIG.3 is the impedance characteristic curves in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION

[0019]FIG.2. is an embodiment of an electro-magnetic interferenceprotection circuit for a clock buffer in accordance with the presentinvention. The electro-magnetic interference protection circuit 200includes a clock buffer 210, a ferrite bead 220, and a capacitor 230.

[0020] The clock buffer 210 receives the clock signal clk-in andbuffer-outputs the clock signal clk-out to drive the transceiver IC (notshown in the figures) in the communication device. Generally, the powersource is the main source of the EMI. For example, in this embodiment,an EMI source has been found by experiment in the power source of theclock buffer 210.

[0021] A ferrite bead 220 is connected to the power source terminal Vccto protect the clock buffer from EMI at 125 MHz.

[0022] To achieve the best result, the ferrite bead 20 having a maximumimpedance at 125 MHz will be used. Referring to FIG.3, the curve A isthe impedance characteristic curve for the ferrite bead 220 in thisembodiment (Model No.

[0023] MLB-201209-450B-N3). Because curve A has a maximum impedance at125 MHz, the ferrite bead 220 corresponding to curve A can achieve thebest result to protect the clock buffer 210 from EMI at 125 MHz.

[0024] Furthermore, to prevent the clock signal clk-out from distortionby the ferrite bead 220, the capacitor 230 is connected in parallel withthe ferrite bead 220. To achieve the best result, the capacitor 230having a minimum impedance at the frequency of the clock signal clk-outwill be used. In this embodiment, the frequency of clk-out is 25 MHz.Hence, a 4.7 μF capacitor is used because its impedance is at a minimumat 25 MHz as showing in curve B of FIG.3. The resultant impedancecharacteristic curve of the ferrite bead 220 and the capacitor 230 isshown in curve C of FIG.3.

[0025] Accordingly, the present invention at least has the followingadvantages.

[0026] First, the clock buffer is protected from EMI.

[0027] Secondly, the communication device using the clock buffer canwork properly.

[0028] Thirdly, the cost for protection from EMI is less because thepresent invention prevents the main EMI source (the power source) frominterfering the clock buffer.

[0029] The above description provides a full and complete description ofthe preferred embodiments of the present invention. Variousmodifications, alternate construction, and equivalent may be made bythose skilled in the art without changing the scope or spirit of theinvention. Accordingly, the above description and illustrations shouldnot be construed as limiting the scope of the invention which isde-fined by the following claims.

1. An electro-magnetic interference protection circuit for a clockbuffer, comprising: a clock buffer, for receiving a clock signal andbuffer-outputting said clock signal; a ferrite bead, wherein saidferrite bead is connected in serial with a power source of said clockbuffer to protect said clock buffer from electro-magnetic interference;and a capacitor, wherein said capacitor is connected in parallel withsaid ferrite bead to reduce the distortion of said clock signal due tosaid ferrite bead.
 2. The electro-magnetic interference protectioncircuit for a clock buffer of claim 1, wherein an impedance of saidferrite bead is at a maximum at a frequency of said electro-magneticinterference.
 3. The electro-magnetic interference protection circuitfor a clock buffer of claim 1, wherein a frequency of saidelectro-magnetic interference is 125 MHz.
 4. The electro-magneticinterference protection circuit for a clock buffer of claim 1, whereinan impedance of said capacitor is at a minimum at a frequency of saidclock signal.
 5. The electro-magnetic interference protection circuitfor a clock buffer of claim 1, wherein a frequency of said clock signalis 25 MHz.
 6. An electro-magnetic interference protection method for aclock buffer, comprising the steps of: connecting a ferrite bead inserial with a power source of said clock buffer to protect said clockbuffer from electro-magnetic interference; and connecting a capacitor inparallel with said ferrite bead to reduce the distortion of a clocksignal outputting by said clock buffer due to said ferrite bead.
 7. Theelectro-magnetic interference protection method for a clock buffer ofclaim 6, wherein an impedance of said ferrite bead is at a maximum at afrequency of said electro-magnetic interference.
 8. The electro-magneticinterference protection method for a clock buffer of claim 6, wherein afrequency of said electromagnetic interference is 125 MHz.
 9. Theelectro-magnetic interference protection method for a clock buffer ofclaim 6, wherein an impedance of said capacitor is at a minimum at afrequency of said clock signal.
 10. The electro-magnetic interferenceprotection circuit for a clock buffer of claim 6, wherein a frequency ofsaid clock signal is 25 MHz.